Many applications, such as gas chromatography applications, require a controlled source of pressure and/or flow of a fluid in order to obtain accurate and repeatable measurements. Pressure regulators are commonly used to maintain the pressure of a fluid in a test device at a constant target pressure to ensure accurate and reproducible measurements. Flow controllers, also referred to as mass flow controllers, are used to control the flow of a fluid, such as a gas or a liquid, through a test device, such as a gas chromatograph.
A typical pressure regulator includes a chamber having a regulated pressure. The regulated chamber is in fluid communication with the test device so that the pressure in the test device is regulated along with the pressure in the chamber. The pressure regulator also includes a source of pressurized gas and a valve for releasing the pressurized gas into the chamber. The source of pressurized gas is used to raise the pressure in the chamber when the pressure falls below the target pressure. A mass flow controller uses a similar valve to control and regulate the flow through the valve using, for example, a flow measurement as feedback to control the flow through the valve.
Some valves have a cylindrical seal and some valves have an o-ring seal that controls the flow of gas from the source to the chamber. Such valves can also be referred to as a variable fluid flow restrictor. The seal is normally biased against a valve seat when the chamber is above the target pressure. When the pressure in the chamber falls below the target pressure, the bias against the seal is released, and the seal separates from the valve seat, allowing the pressurized gas to flow into the chamber.
A disadvantage to these types of valves is that when the seal separates from the valve seat, it does not maintain a stable position. Instead, sections of the seal may actually oscillate between separation and contact with the valve seat. The material of the valve seal may also stick to the surface of the valve seat, a condition referred to as “stiction.” Stiction between the valve seal and the surface of the valve seat may cause a hysteresis effect when the valve attempts to open, sticks, and then opens further than desired. Once the valve opens further than desired, the valve may exceed its setpoint and then immediately close to compensate, thus producing an undesirable hysteresis effect. Such hystersis makes the valve difficult to control. Regardless of the manner caused, oscillation of the seal creates disturbances in the flow of pressurized gas, which causes unstable gas flow through the valve and pressure anomalies in the chamber receiving the pressurized gas. Because the test device is in fluid communication with the chamber, the pressure within the test device fluctuates, which negatively affects measurements taken in the test device.
Accordingly, a need exists for a flow control capable of providing stable fluid flow.